Fifty Years of Quarks!

Today, physicists are both excited and disturbed by how well the Standard Model is behaving, even at the enormous energies provided by the CERN Large Hadron Collider. There seems to be no sign of the expected supersymmetry property that would show the way to the next-generation version of the Standard Model: Call it V2.0. But there is another ‘back door’ way to uncover its deficiencies. You see, even the tests for how the Standard Model itself works are incomplete, even after the dramatic 2012 discovery of the Higgs Boson! To see how this backdoor test works, we need a bit of history.

Over fifty years ago in 1964, physicists Murray Gell-Mann at Caltech and George Zweig at CERN came up with the idea of the quark as a response to the bewildering number of elementary particles that were being discovered at the huge “atom smasher” labs sprouting up all over the world. Basically, you only needed three kinds of elementary quarks, called “up,” “down” and “strange.” Combining these in threes, you get the heavy particles called baryons, such as the proton and neutron. Combining them in twos, with one quark and one anti-quark, you get the medium-weight particles called the mesons.

This early idea was extended to include three more types of quarks, dubbed “charmed,” “top” and “bottom” (or on the other side of the pond, “charmed,” “truth” and “beauty”) as they were discovered in the 1970s. These six quarks form three generations — (U, D), (S, C), (T, B) — in the Standard Model.

Particle tracks at CERN/CMS experiment (credit: CERN/CMS)

Early Predictions

At first the quark model easily accounted for the then-known particles. A proton would consist of two up quarks and one down quark (U, U, D), and a neutron would be (D, D, U). A pi-plus meson would be (U, anti-D), and a pi-minus meson would be (D, anti-U), and so on. It’s a bit confusing to combine quarks and anti-quarks in all the possible combinations. It’s kind of like working out all the ways that a coin flipped three times give you a pattern like (T,T,H) or (H,T,H), but when you do this in twos and threes for U, D and S quarks, you get the entire family of the nine known mesons, which forms one geometric pattern in the figure below, called the Meson Nonet.

The basic Meson Nonet (credit: Wikimedia Commons)

If you take the three quarks U, D and S and combine them in all possible unique threes, you get two patterns of particles shown below, called the Baryon Octet (left) and the Baryon Decuplet (right).

Normal baryons made from three-quark triplets

The problem was that there was a single missing particle in the simple 3-quark baryon pattern. The Omega-minus (S,S,S) at the apex of the Baryon Decuplet was nowhere to be found. This slot was empty until Brookhaven National Laboratory discovered it in early 1964. It was the first indication that the quark model was on the right track and could predict a new particle that no one had ever seen before. Once the other three quarks (C, T and B) were discovered in the 1970s, it was clear that there were many more slots to fill in the geometric patterns that emerged from a six-quark system.

The first particles predicted, and then discovered, in these patterns were the J/Psi “charmonium” meson (C, anti-C) in 1974, and the Upsilon “bottomonium” meson (B, anti-B) in 1977. Apparently there are no possible top mesons (T, anti-T) because the top quark decays so quickly it is gone before it can bind together with an anti-top quark to make even the lightest stable toponium meson!

The number of possible particles that result by simply combining the six quarks and six anti-quarks in patterns of twos (mesons) is exactly 39 mesons. Of these, only 26 have been detected as of 2017. These particles have masses between 4 and 11 times more massive than a single proton!

For the still-heavier three-quark baryons, the quark patterns predict 75 baryons containing combinations of all six quarks. Of these, the proton and neutron are the least massive! But there are 31 of these predicted baryons that have not been detected yet. These include the lightest missing particle, the double charmed Xi (U,C,C) and the bottom Sigma (U, D, B), and the most massive particles, called the charmed double-bottom Omega (C, B, B) and the triple-bottom omega (B,B,B). In 2014, CERN/LHC announced the discovery of two of these missing particles, called the bottom Xi baryons (B, S, D), with masses near 5.8 GeV.
To make life even more interesting for the Standard Model, other combinations of more than three quarks are also possible.

Exotic Baryons
A pentaquark baryon particle can contain four quarks and one anti-quark. The first of these, called the Theta-plus baryon, was predicted in 1997 and consists of (U, U, D, D, anti-S). This kind of quark package seems to be pretty rare and hard to create. There have been several claims for a detection of such a particle near 1.5 GeV, but experimental verification remains controversial. Two other possibilities called the Phi double-minus (D, D, S, S, anti-U) and the charmed neutral Theta (U, U, D, D, anti-C) have been searched for but not found.

There are also tetraquark mesons, which consist of four quarks. The Z-meson (C, D, anti-C, anti-U) was discovered by the Japanese Bell Experiment in 2007 and confirmed in 2014 by the Large Hadron Collider at 4.43 GeV, hence the proper name Z(4430). The Y(4140) was discovered at Fermilab in 2009 and confirmed at the LHC in 2012 and has a mass 4.4 times the proton’s mass. It could be a combination of charmed quarks and charmed anti-quarks (C, anti-C, C, anti-C). The X(3830) particle was also discovered by the Japanese Bell Experiment and confirmed by other investigators, and could be yet another tetraquark combination consisting of a pair of quarks and anti-quarks (q, anti-q, q, anti-q).

So the Standard Model, and the six-quark model it contains, makes specific predictions for new baryon and meson states to be discovered. All totaled, there are 44 ordinary baryons and mesons that remain to be discovered! As for the ‘exotics’ that opens up a whole other universe of possibilities. In theory, heptaquarks (5 quarks, 2 antiquarks), nonaquarks (6 quarks, 3 antiquarks), etc. could also exist.

At the current pace of a few particles per year or so, we may finally wrap up all the predictions of the quark model in the next few decades. Then we really get to wonder what lies beyond the Standard once all the predicted particle slots have been filled. It is actually a win-win situation, because we either completely verify the quark model, which is very cool, or we discover anomalous particles that the quark model can’t explain, which may show us the ‘backdoor’ way to the Standard Model v.2.0 that the current supersymmetry searches seem not to be providing us just yet.